The invention relates to the field of repair of a cast article, and more particularly to repair of a cast article having imperfections.
Certain articles, such as airfoils for the power turbine sections of gas turbines, are constructed of superalloys. Due to the extreme temperatures and stresses to which such cast superalloy articles such as, airfoils, are exposed, imperfections in cast articles can have serious consequences. In the case of an airfoil, the control of the airfoil wall's thickness, when an airfoil is cast, is critical to the strength and life of the airfoil.
More particularly, turbine components such as turbine airfoils are typically cast as hollow structures with complex cooling passages inside. The complex cooling passages are desired because the temperatures of the hot combustion gases directed at the airfoil during operation are at or above the melting temperature of the superalloy composition. The cooling medium may comprise air or steam. In the case of steam, such steam will typically be under pressure higher than cooling air pressure. The airfoil design is sensitive to airfoil wall thickness control because of these factors. Insufficient thickness results in significant creep damage due to high local stresses in areas of reduced thickness (i.e. cross-sectional area), while excessive wall thickness results in fatigue damage due to insufficient cooling of the exterior. An about 0.08" nominal airfoil thickness is predicted to result in an airfoil achieving its design life if actual thickness can be maintained to within about 0.02" of the nominal thickness.
The directional casting process for superalloy components, such as airfoils, typically utilizes ceramic "bumpers" on core surfaces or mold surfaces. For a nominal cast wall thickness of about 0.08"+/-0.2", ceramic bumpers of about 0.06" in eight help control wall thickness by the geometric constraints provided in the form of the ceramic bumpers. A large number of bumpers on each casting reduces the likelihood of distortion of the ceramic molds and cores.
After the article is directionally cast, the ceramic bumpers are removed. The wall of the article will be thin wherever there had been ceramic bumpers. In addition, if the mold and the core had been in contact, there would be a small hole in the article at a contact point, and a corresponding thicker area on the other side of the article. Because variations in wall thickness beyond the design specification of about 0.08"+/- about 0.02" may result in early failure of the airfoil, restoration of wall thickness is needed.
The restoration of wall thickness is complicated by the article geometry where, for example, an airfoil. At every airfoil bumper location there is a unique combination of inner surface and outer surface contours to the airfoil wall exist. In addition, due to variations in wall thickness within the design specification, no two airfoils will have the same wall thickness profile around the airfoil perimeter or along the airfoil length. An airfoil design can, for example, allow for filler material to project into the airfoil cooling cavity up to about 0.02" before such filler material results in any detrimental perturbation of the coolant flow. Because the coolant can comprise steam, the inner contour at each repair should be reasonably smooth so as not to set up a site for crevice corrosion. Outer surface contours are less of a concern, since the outer surface is easily available for hand blending of the filler material back to the airfoil contour.
In addition to a need to satisfactorily repair articles due to dimensional imperfections resulting from the casting process, a further need to repair articles whereby cracks may have developed at a location on the article during the casting process exists. As well, cracks can develop in airfoils after they have been in service for a period of time. Up to now, once a crack or imperfection of sufficient size has been detected, the airfoil would be immediately replaced. If the crack or imperfection could be repaired in such a way that the airfoil could thereafter withstand the extreme operating conditions in a turbine, then the expense of replacing the cracked airfoil with a new airfoil can be avoided.
In addition to dimensional concerns for the repairing of an airfoil wall at weakened locations, the strength of the airfoil should be maintained. Steam can be a cooling medium for an airfoil, and the steam injected into the cooling passages under pressure creates a pressure vessel effect therein. A repair should be able to withstand the interior pressure and be leak-tight, and also be almost as strong as the surrounding directional material in terms of strength and resistance to creep and fatigue.
Airfoils are typically formed of superalloys that can have directionally oriented microstructures to satisfy the mechanical strength demands for creep resistance or fatigue resistance to achieve a satisfactory design life. Directionally oriented microstructures can be produced using directional solidification processes, which result in either elongated polycrystalline grains or single crystals. Directionally oriented microstructures may be problematic to the repair of dimensional imperfections and cracks therein, since the repaired area should have a similarly oriented microstructure of uniform strength.
Accordingly, a need exists for a method to repair a cast article having an area of imperfection in the form of a non-dimensional wall thickness. A further need exists for a method to repair a cast article having an area of imperfection, in which the repaired article in the repaired area of imperfection is capable of withstanding stresses which are substantially equal to, or a substantial amount of, the stresses which would previously be capable of being withstood in absence of such imperfection.
Further, a need exists for a method to repair a cast article, such as a hollow airfoil, having an area of imperfection or a crack, wherein the airfoil, at the repaired area of imperfection or crack, is capable of withstanding interior pressure. Still further, a need exists for a method to repair a cast article, such as a hollow airfoil, having a directionally oriented microstructure and growth axis and an area of imperfection, in which such airfoil, in the repaired area of imperfection, has an identically aligned growth axis that permits the repaired area to withstand stresses a substantial portion of those previously capable of being withstood by the airfoil.